Fluid level sensor

ABSTRACT

A fluid level or fluid volume sensor or probe for mounting in a fluid storage vessel for sensing the level or volume of the fluid within the vessel which is comprised of four like electrically conductive capacitor elements each formed to present two electrically connected capacitive plates disposed in angular relation to each other. Electrical insulating material constitutes centrally located means for mounting the capacitor elements in fixed spaced relationship from one another around such mounting means and for positioning the elements so that each capacitive plate thereof defines with a capacitive plate of the next adjacent capacitor element a dielectric space therebetween whereby the mounted capacitor elements together form four like dielectric spaces. Material of known constant dielectric value fills two of the dielectric spaces thereby forming with their respective space defining capacitive plates two capacitors of known fixed and substantially like capacitive value. The remaining two dielectric spaces are open to receive varying levels of fluid thereby forming with their respective capacitive plates, and the fluid within the spaces, two capacitors of variable capacitive value. The sensor or probe is not limited in its application to fluid level or fluid volume measurements of liquids in vessels and may be used for volume measurement with respect to a wide variety of multi-component systems (including liquid/liquid, liquid/gas, liquid/solid, solid/gas, etc.) as long as the dielectric constants for the two or more components are different.

CROSS REFERENCE TO RELATED APPLICATION

The present invention is related to my commonly assigned U.S. PatentApplication Ser. No. 345,350 filed Feb. 3, 1982, entitled METHOD OFMEASURING AND INDICATING FLUID LEVELS.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to systems for measuring and indicatingthe level and/or quantity of a fluid in a container or storage tank andis particularly applicable to the problem of accurately measuring thelevel or amount of fuel in vehicular fuel tanks including fuel tanksused in aircraft, watercraft and landcraft. In a more directive sense,the invention relates to fluid level or fluid quantity measuring andindicating systems which include a capacitive type immersion sensor orprobe.

2. Description of the Prior Art

Many techniques have been developed to measure the liquid levels andliquid quantities in storage tanks. The most common system for measuringthe fuel level in the fuel tank of a motor vehicle employs a variableresistor within the tank. The wiper arm of the variable resistor isconnected through a pivot to a float which monitors the upper level ofthe fuel in the tank. When the vehicle is traveling on a grade, the fuelsurface level is shifted at an angle to its normal horizontal referenceplane within the fuel tank and causes the float to monitor an erroneouslevel, either higher or lower than the correct level, a phenomenontermed "fuel level shift." Additionally, when the vehicle starts,accelerates, slows or stops its motion, waves are generated in thestored fuel. This phenomenon is commonly referred to as "sloshing" andcauses the float to bounce up and down, thereby affecting the fuelmeasurement readings. Mechanical and electrical damping techniques havebeen employed to reduce the effects of fuel level shift and sloshing,but they have not been shown to be suitable for obtaining instantaneousand accurate fuel level measurements. The adverse affects of fuel levelshift and sloshing are even more troublesome in the fuel tanks ofaircraft and high speed landcraft and watercraft.

To overcome the problems of fuel level shift and sloshing in fuel levelmeasuring and indicating, a number of systems have been proposed whichuse immersion condenser units or probes in liquid containers or tanks incombination with suitable electrical circuitry and measuring andindicating instrumentation. In U.S. Pat. No. 2,357,023, granted to O.W.Reid et al and entitled "Liquid Level Measuring Apparatus," theinventors disclose the use of a plate type immersion condenser unitlocated in a liquid container and for which the liguid itself forms thedielectric between the plates. The capacitive value of the immersioncondenser is variable and changes with respect to the level of liquid inthe container. By constructing the condenser unit of multiple pairs ofplates which are placed in different areas of the container the effectsof liquid sloshing or surging and liquid level shift are reduced so asto obtain a more accurate capacitance value for the immersion unit. Theimmersion condenser unit (whether of single pair or multiple pair platedesign) is electrically connected into an alternating current bridgecircuit having two condensers of fixed and equal capacity each formingan arm of the bridge, the remaining two arms of the bridge being made upof a variable capacity condenser and the immersion condenser. The bridgecircuit (of well known Wheatstone configuration) is connected to asource of alternating current of predetermined frequency and detectorand measurement indicating circuitry. The bridge circuitry is arrangedto be in an unbalanced state so long as any liquid remains in thecontainer in the dielectric space between the plates of the immersioncondenser unit.

U.S. Pat. No. Re. 23,493, granted to A. Edelman and entitled "LiquidLevel Gauge" also discloses liquid level detection and measurementindicating circuitry incorporating a plate type immersion condenserunit. This unit, termed a "measuring condenser," has a capacitive valuewhich changes with respect to the level of liquid in a container ortank. The circuitry also includes a "comparison condenser" unit of platetype design which is always maintained fully immersed in the liquid tobe measured as to its level or volume. The comparison condenser withrespect to different liquids (having different dielectric values) isvariable in its capacitance value but with respect to the measuringcondenser, and its sensing of various levels of like liquid, provides acompensating or comparison capacitive value to the circuitry so thatmeasurement of a liquid level is independent of the dielectric constantof the liquid and any variation thereof. Both the measuring condenserunit and the comparison condenser unit are exposed to substantially thesame ambient conditions as the liquid being measured.

In U.S. Pat. No. 4,194,395, granted to T. J. Wood and entitled"Capacitive Liquid Level Sensor," a capacitive type sensor for measuringliquid levels is proposed in which a plurality of like plate typecapacitors are aligned in parallel. The dielectric spaces of eachcapacitor (isolated from each other) receive the liquid to be measuredwhich (with air, if any, above the liquid) establishes the dielectricfor the spaces and thus the capacitance value of each capacitor. Sincethe capacitors are identically configured they exhibit equal values ofcapacitance only when the liquid dielectric (and air, if any) betweenthe plates of each capacitor covers equal areas. When the liquid withina container is being measured as to its height level or volume and issloshing or has its level disoriented with respect to its normal liquidlevel reference plane, the liquid (functioning as a dielectric) coversdifferent areas of the capacitors and they exhibit dissimilar values ofcapacitance. Associated circuitry interrogates the capacitors and atpoints when the capacitance values approach equality the system readsone of the values and registers the liquid level or quantity of liquidremaining in the container.

SUMMARY OF THE INVENTION

Prior art capacitive liquid level sensors and associated interrogating,balancing, interpreting and measurement indicating circuitry are complexand deficient in their approaches to solving the problems of liquidsloshing and liquid level shift and the effects on liquid level andvolume measurement of changes in the physical and chemicalcharacteristics of the liquid being measured and of the multiplecharacteristics of the environment of the liquid and its container. Thepresent invention addresses the complexities of the prior art andprovides fluid level or volume measurements with a high degree ofaccuracy through a unique capacitive sensor or probe of multi-capacitordesign and simplified associated circuitry which is insensitive toenvironmental changes and stray capacitances.

It is an object of the present invention to provide an improvedcapacitive type sensor applicable to liquid level or volume measurementin both stationary and vehicular liquid storage containers and tanks.

It is another object of the present invention to provide a capacitivetype liquid level sensor of multi-capacitor design that accuratelydetects and measures liquid levels or volumes in liquid storage tankswhen the liquid therein is sloshing and/or misoriented with respect toits normal surface plane of reference.

A still further object of the present invention is to provide acapacitive type fluid level sensor of multi-capacitor design that isrelatively insensitive in its accuracy of measurement to changes in theenvironmental characteristics of such fluid and its container.

Another object of the present invention is to provide a capacitive typefluid level sensor or multi-capacitor design in association with asimple alternating current bridge circuit, including detector and directreadout circuitry, which is insensitive to changes in the environmentalcharacteristics of such fluid and its container, to fluid motion andmisorientation of the container, or to stray capacitance in thesensor-bridge system.

The present invention is intended as an improvement to conventionalfluid level or volume measurement systems and is broadly suitable foruse in all types of fluid storage containers and tanks and for use withrespect to fluid level or volume measurement of conducting as well asnon-conducting fluids. The system is also applicable to level and volumemeasurement of conducting and non-conducting fluids which are comprised,at least in part, of a liquid material.

It is to be noted and understood that, throughout this specification andthe appended claims, the term or word "fluid" shall and does mean auni-component or multi-component substance or composition which tends toflow or tends to conform to the shape or configuration of its containerand which may exhibit electrically non-conducting or electricallyconducting characteristics. Thus, the term "fluid" encompasses (withoutlimitation) a wide variety of: liquids, gases, powdered or granulatedsolids, liquid/liquid mixtures or emulsions, liquid/gas mixtures ordispersions, liquid/solid mixtures, and gas/solid mixtures. In furtherdefinition of the term "fluid," it is to be understood thatmulti-component substances comprising a fluid must exhibit for eachcomponent a different and determinable dielectric value. Thus, formulti-component fluids or fluids comprised of different phases of thesame substance, measurable differences must be exhibited with respect tothe dielectric constants for such components or phases.

For purposes of ease of description of the invention and its applicationto fluid level and fluid quantity measurement, the sensor designcomprising the invention will, for the most part, be discussed in termsof its applicability to liquid level or liquid volume measurement.

The invention includes a uniquely configured capacitive sensor ormeasurement probe structure which extends from the top or high point ofa fluid storage tank, in its usual orientation, to the bottom or lowpoint of such tank in such orientation. The capacitive sensor iscomprised of four plate type capacitors extending in clustered parallelalignment throughout the length of the sensor so that each capacitor isin contact with the stored fluid throughout the range of levels to bemonitored and all capacitors are exposed to the same fluid, atmosphericand container enviroment. The four capacitors are constructed from fourelectrically conductive capacitor elements each comprised of twoelectrically connected capacitive plates. The capacitor elements aremounted in fixed spaced relationship from one another and positioned bythe mounting means so that each plate of each capacitor element defineswith a plate of the next adjacent capacitor element a dielectric spacetherebetween whereby the mounted capacitor elements together form fourdielectric spaces. Material of constant dielectric value fills two ofthe dielectric spaces thereby forming with the respective space definingplates a pair of capacitors of fixed capacitive value. The other twodielectric spaces remain open to receive variable quantities of thefluid to be sensed and the plates defining such spaces with the variablequantities of fluid therein cooperate to form a pair of variablecapacitors.

In the preferred structure of the multi-capacitor sensor of the presentinvention, the two fixed value capacitors are of equal capacitance valueand are identically constructed and configured with respect to eachother so that they respond in like fashion to the environment of thefluid being measured and its container. The two variable valuecapacitors are likewise identically constructed and configured withrespect to each other so that they too respond in like fashion to theenvironment of the fluid and its container. Since the two variable valuecapacitors are identically configured they exhibit equal values ofcapacitance only when the fluid (having its specific dielectric value)between the plates of each capacitor fills equal spaces and covers equalareas. As a corollary, when the fluid within the container is sloshingor has its level disoriented with respect to its normal plane of reposein the container, the fluid functioning as a dielectric in the twovariable capacitors will fill different space volumes and coverdifferent areas of such capacitors and they will exhibit dissimilarvalues of capacitance.

The unique structure of the capacitive sensor, as described in detailhereinafter, permits the pair of fixed value capacitors (having likecapacitive value) and the pair of variable value capacitors to bedirectly utilized as the four capacitive legs or sides of a classicalternating current Wheatstone bridge circuit having an alternatingcurrent generator (constant voltage at set frequency) and associatedcurrent detection, measurement and value indicating instrumentcircuitry. The bridge circuitry, comprised of the two fixed capacitorsand two variable capacitors forming the sensor, is arranged to be in anunbalanced state (current flowing through the detection circuit) so longas any liquid remains in the container in the dielectric spaces betweenthe plates of the variable capacitors. With the bridge structured andoperating in this fashion the detector circuitry reads the bridgeunbalance (value of current flow) linearly as a direct measurement offluid level or volume of fluid in the container within which the sensoris placed. Because all capacitors of the bridge circuit are part of andcomprise the multi-capacitor sensor in the system, the bridge circuit isinsensitive to stray capacitance and such bridge circuit capacitorcomponents are all subjected to like environmental conditions.

The two variable capacitors of the sensor unit are directly affected intheir capacitance values by the fluid levels therein at close butseparate locations and the associated detection circuitry may bedesigned to interrogate these sensor capacitors to derive an outputcharacteristic value at the instants at which the fluid levels withinsuch capacitors are equal (capacitive values then are equal). At suchinstances the unbalanced state of bridge (value of current flow) ismeasured and the value indicating circuitry reports (visual and/orrecorded) the exact fluid level or volume value in direct linearrelationship to the current value. Thus, the unique sensor or probe ofthis invention monitors and measures fluid levels and volumes withextraordinary accuracy and communicates such measurements through simplealternating currect bridge and associated circuitry for indicatingand/or recording in direct digital readout display or analog display,and/or printed fashion or as an electrical signal for control or otherpurposes. The system circuitry may be designed to hold the preceedinglevel or quantity measurement value until the generation of a new levelor quantity value reading is made.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away view of a liquid storage tank in which amulti-capacitor fluid level sensor of the present invention is mounted.

FIG. 2 is a cross-sectional view of the sensor shown in FIG. 1.

FIG. 3 is an electrical diagram of the bridge circuitry, including themulti-capacitor probe of FIG. 1, employed in the fluid level and volumemeasuring system of the present invention.

FIG. 4 is a schematic/block diagram of the circuitry of FIG. 3 in whichthe capacitors of the sensor are more clearly identified in a classicWheatstone bridge circuit configuration.

FIG. 5 is a cross-sectional view of an alternative fluid level sensor ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A multi-capacitor liquid level sensor 10, in accordance with the presentinvention, is illustrated in FIG. 1 in installed position within aliquid storage tank 11 (having a top wall 12, side wall 13 and bottomwall 14) containing a quantity of liquid 15. As shown, the sensor 10extends from the bottom of the tank to the top and is generally orientedso that the vertical axis of the sensor is normal to the surface planeof the liquid in its static condition. For other and more complex tankconfigurations the sensor need not be mounted in vertical orientationwith respect to the surface plane of static fluid so long as the sensorspans the full range of fluid levels experienced within the tank.

The liquid level sensor 10 of FIG. 1 (shown in cross-sectional view inFIG. 2) is comprised of four electrically conductive capacitor elements16, 17, 18, and 19 each comprised of two capacitive plates "A" and "B".A non-conductive element 20 is provided for mounting the capacitorelements in fixed spaced relationship from one another and forpositioning such elements so that each plate thereof defines with aplate of the next adjacent capacitor element a dielectric spacetherebetween whereby the four mounted capacitor elements together formfour dielectric spaces. As shown in FIGS. 1 and 2 the non-conductivemounting element 20 extends between plate A of element 16 and plate B ofelement 17 and between plate A of element 18 and plate B of element 19and fills the dielectric spaces between these pairs of plates. Thus,non-conductive mounting element 20, comprised of a sheet of dielectricmaterial, forms with conductive plate A of element 16 and conductiveplate B of element 17 and with conductive plate A of element 18 andconductive plate B of element 19 two plate-type capacitors C₂ and C₃ oflike fixed capacitance (impedance) value.

The dielectric spaces 21 and 22, defined (respectively) by plate B ofelement 16 and plate A of element 19 and plate B of element 18 and plateA of element 17, remain open and free to receive variable quantities andlevels of the liquid to be sensed together with vapors above suchlevels. Thus, the variable liquid-vapor quantities within spaces 21 and22 form with conductive plate B of element 16 and conductive plate A ofelement 19 and conductive plate B of element 18 and conductive plate Aof element 17 two plate-type capacitors C1 and C₄ of variablecapacitance (impedance) value.

The multi-capacitor sensor 10 is insulated from the tank by any wellknown means. Electrically conductive lead wires L₁, L₂, L₃ and L₄ areconnected, respectively, to capacitor elements 16, 17, 18 and 19 andleave the tank 11 via insulated passage therefrom. When these lead wiresare further connected to alternating current generator circuitry "e" anddetection, measurement and value indicating circuitry "D," as shown inFIG. 3, there results simple bridge circuitry of classic Wheatstoneconfiguration as shown schematically in FIG. 4.

The alternating current generator circuitry e is connected to the bridgecircuitry (the capacitors C₁, C₂, C₃ and C₄ of the sensor 10) throughscreened input lead wires L₂ and L₄. Outlet wires (screened) L₁ and L₃connect the bridge circuitry to the detector circuitry D (includingappropriate current rectifier circuitry, if required), bridge signalinterrogation circuitry, current measurement circuitry and current valueindicating circuitry. The bridge signal interrogation circuitry may bedesigned to respond only to instances when the capacitance values of thevariable value capacitors C₁ and C₃ of the sensor are equal (instanceswhen these capacitors contain equal fluid levels). At such instances thecurrent measurement circuitry determines the output current value of thebridge and the current value indicating circuitry translates such outputvalue into a readout volmetric or fluid level value or recordedvolumetric or fluid level value. Alternatively, the frequency or voltageof the applied alternating current may be varied until the detectorcircuitry (including its measuring and/or value indicatinginstrumentation) is set at a predetermined point, the liquid volumebeing then read off directly or recorded in gallons, liters or othervolumetric units by sensing the varied frequency (or its period) orvoltage, respectively.

For the bridge circuit of FIG. 4 the respective capacitor may haveimpedance value as indicated below:

Fixed value capacitors

C₂ impedance=Z₂

C₃ impedance=Z₃

Variable value capacitors

C₁ impedance=Z₁

C₄ impedance=Z₄

The detector circuitry D also presents an impedance value which may bedesignated as Z₅. The alternating current generator circuitry, at setfrequency, has a constant voltage "e" which is applied across thebridge. Thus,

If Z₂ =Z₃ and Z₁ =Z₄, and

If Z₅ is of small value, i.e., Z₅ <Z₁ and Z₂ then it can be establishedthat the current "i" through the detector circuitry is: ##EQU1## where:e is the voltage value and

w=2π× frequency

The current value i in the detector circuitry changes in linear relationto changes in the capacitance value of capacitor C₁. Therefore, changesin the C₁ -C₂ relationship may be determined by measuring changes in thecurrect value i. Furthermore, with appropriate detector and a.c.generator circuitry (e.g. by keeping the current i constant and varyingthe w value) the readout of the C₁ -C₂ value can be effected bymeasuring the period of the angular frequency w. Thus, the readoutintrumentation in the detector circuitry may yield digital values indirect linear relationship to the actual liquid volume values within thetank or container wherein the sensor of this invention is mounted.

It should be understood, that the multi-capacitor fluid level sensingmethod of the invention is not limited to use with non-conductingfluids. It is equally practical to make fluid level or volumemeasurements on conducting fluids, provided that the capacitive platesof the electrically conductive capacitor elements of the sensor arecoated with an insulating film thereby preventing the passage ofshorting components of current between such plates via the conductingfluids. Thus, there is shown in cross-sectional view in FIG. 5 a fluidlevel sensor 30, of design similar to the sensor 10 of FIGS. 1, 2 and 3,including electrically conductive capacitor elements 36, 37, 38, and 39mounted in fixed spaced relationship by non-conductive element 40. Thefour conductive capacitor elements are coated with an insulating flim 43which maintains the capacitance integrity of the pair of fixed valuecapacitors and pair of variable value capacitors comprising the sensor30.

The unique multi-capacitor sensor of the present invention also is notlimited to level or volume measurements of liquids in tanks or storagecontainers, but may be used (for example) in the level or volumetricmeasurement of powdered and granular materials in tanks, containers,hoppers and the like. Broadly, the sensor may be used for volumemeasurement with respect to a wide variety of multi-component systems(liquid/liquid, liquid/gas, liquid/solid, solid/gas, etc.) as long asthe dielectric constants for the two or more components are different.It must be understood that in all level or volumetric measurement usesof the sensor a multi-component system (at least two materials) isinvolved over the measurement range. Thus, in the case of a simple fueltank only at two measured instances is a single component sensed, i.e.,when the tank is absolutely filled with fuel of known dielectric valuewith no air present and when the tank is absolutely empty of fuel withonly air present with its known dielectric value. All other sensedinstances involve the measurements of the dielectric value of amulti-component system, i.e., the fuel and air. The only requirement forapplicability of the sensor for depth or volumetric measurement of amulti-component system is that the two dielectric spaces of the variablevalue capacitors of the sensor be exposed to the system over the entirerange of component variety or change for which measurement is desired.

Powdered and granular materials in containers and hoppers, as mentionedabove, also present a multi-component system, the depth or volume ofwhich can be measured by the method of this invention. Such materialshave a determinable dielectric value in their normal state of gravitypacked repose. Thus, as a stored material or hopper fed material, volumeor depth measurements can be made with the method for a two-componentsystem comprised of the material and air. Further, the volume of suchsolid materials in a fluidized state (solid/gas mixture) can beascertained by the method.

In a more complex application of the method the volume of liquids in agas can be measured and reported. Thus, with proper calibration of the"full" and "empty" settings of the detector circuitry of the bridgecircuit associated with a sensor (with insulating coating on capacitorplates) mounted in a water storage tank, the volume of water in the tankcan be accurately measured even though the water therein is subjected tozero gravity and is suspended as micro-droplets in the air (liquid/gasmixture) within the tank. The measures the combined dielectric value ofthe dispersion of water droplets and air.

Numerous other examples of multi-component systems can be suggested forwhich depth and/or volume measurements can be made and reported via themulti-capacitor sensor of the present invention and the simpleWheatstone bridge circuitry associated therewith. The sensor system isinsensitive to stray capacitances because it contains all of the fixedand variable capacitors comprising the bridge circuitry and all of suchcapacitors are subject to the same multi-component system and containerenviroment. Further, the sensor system may be applied to material level,depth and volume measurement situations in which the material containeris subjected to misorientation and the material within the container issubjected to sloshing or other unnatural environmental conditions suchas low or high temperatures or zero gravity.

While the invention has been described in detail with respect to anumber of preferred embodiments, it will be understood by those skilledin the art that various changes and modifications may be made withoutdeparting from the spirit and scope of the novel concept of thisinvention. Therefore, it is intended by the appended claim to cover allsuch modifications and variations which fall within the true spirit andscope of the invention.

What is claimed is:
 1. A capacitive type fluid level sensor for mountingin a fluid storage vessel for sensing the level of the fluid within saidvessel over a predetermined height range comprising:(a) fourelectrically conductive capacitor elements each formed to present twoelectrically connected capacitive plates, said elements having a lengthat least equal to said height range; (b) means for mounting saidcapacitor elements in fixed spaced relationship from one another and forpositioning said elements so that each capacitive plate thereof defineswith a capacitive plate of the next adjacent capacitor element adielectric space therebetween whereby said mounted capacitor elementstogether form four dielectric spaces; and (c) material of known constantdielectric value within two of the dielectric spaces thereby formingwith their respective space defining capacitive plates two capacitors offixed electrical capacitive value,the remaining two dielectric spacesbeing open to receive varying levels of the fluid within said storagevessel thereby forming with their respective capacitive plates twocapacitors of variable electrical capacitive value.
 2. A capacitive typefluid level sensor as defined in claim 1 wherein the two capacitors ofknown fixed electrical capacitive value have substantially likecapacitive value.
 3. A capacitive type fluid level sensor as defined inclaim 1 wherein the four electrically conductive capacitor elements eachconsist of a rectangular metallic sheet folded along a line midway theedges thereof to form said electrically connected capacitive plates. 4.A capacitive type fluid level sensor as defined in claim 1 wherein thefour electrically conductive capacitor elements each consist of ametallic sheet folded 90 ° along a line midway the edges thereof to formsaid electrically connected capacitive plates.
 5. A capacitive typefluid level sensor as defined in claim 4 wherein the mounted andpositioned electrically conductive capacitor elements are of likeconfiguration and form four like dielectric spaces.
 6. A capacitive typefluid level sensor as defined in claim 1 wherein the means for mountingthe capacitor elements in fixed spaced relationship is formed of a solidelectrically insulating material.
 7. A capacitive type fluid levelsensor as defined in claim 6 wherein the means for mounting thecapacitor elements in fixed spaced relationship extends into two of thedielectric spaces as the material of constant dielectric value formingwith the space defining capacitive plates of said spaces the twocapacitors of fixed capacitive value.
 8. A capacitive type fluid levelsensor as defined in claim 1 wherein the four mounted electricallyconductive capacitor elements forming said four dielectric spaces arepositioned so that said spaces are aligned as two diametrically opposedspace pairs and the material of constant dielectric value within two ofsaid spaces is within the spaces forming one of said space pairs.
 9. Acapacitive type fluid level sensor as defined in claim 1 wherein thefour mounted electrically conductive capacitor elements are coated withan insulating film.
 10. A capacitive type fluid level sensor formounting in a fluid storage vessel for sensing the level of the fluidwithin said vessel over a predetermined height range comprising:(a) fourlike electrically conductive capacitor elements each formed to presenttwo electrically connected capacitive plates disposed in angularrelation to each other, said elements and said plates having a length atleast equal to said height range; (b) means formed of electricalinsulating material centrally located for mounting said capacitorelements in fixed spaced relationship from one another around saidmounting means and for positioning said elements so that each capacitiveplate thereof defines with a capacitive plate of the next adjacentcapacitor element a dielectric space therebetween whereby said mountedcapacitor elements together form four like dielectric spaces; and (c)material of known constant dielectric value within two of the dielectricspaces thereby forming with their respective space defining capacitiveplates two capacitors of known fixed and substantially like capacitivevalue,the remaining two dielectric spaces being open to receive varyinglevels of fluid thereby forming with their respective capacitive platesand fluid within said spaces two capacitors of variable capacitivevalue.
 11. A capacitive type fluid level sensor as defined in claim 10wherein the centrally located means for mounting the electricallyconductive capacitor elements in fixed spaced relationship extends intotwo of the dielectric spaces as the material of constant dielectricvalue forming with the space defining capacitive plates of said spacesthe two capacitors of fixed and substantially like value.
 12. Acapacitive type fluid level sensor as defined in claim 10 wherein thefour mounted like electrically conductive capacitor elements formingsaid four like dielectric spaces are positioned so that said sapces arealigned as two diametrically opposed space pairs and the material ofconstant dielectric value within two of said spaces is within the spacesforming one of said space pairs.